It is generally recognized that cholinergic responses in the heart include negative inotropic and chronotropic effects in the atria, slowing of AV conduction and negative inotropic effects in the ventricle. These responses have been attributed to the actions of acetylcholine on an homogeneous population of muscarinic receptors. However, the notion of such an homogeneous population is challenged by the existence of many studies reporting discrepant effects of acetylcholine depending on dose used, tissue studied, parameter measured or prior level of autonomic tone. Recent published evidence from our laboratory has documented distinct differences between muscarinic responses at the sinus node and AV node and further preliminary experiments point toward the possibility of differences in muscarinic receptor sensitivity or specificity in mediating these phenomena. It will be the purpose of this investigation to critically analyze cardiac responses to cholinergic activation to determine if differences in muscarinic receptor sensitivity or specificity could underly the differences in acetylcholine action noted by many investigators. The experiments are designed to compare sinus nodal, AV junctional and idioventricular automaticity in response to cholinergic activation using the technique of selective perfusion of individual cardiac regions. Muscarinic receptor characterization will be achieved using selective and non-selective agonists and antagonists. In addition, AV conduction will be examined in response to cholinergic activation using similar techniques. These experiments are designed to clarity the mechanism(s) contributing to the reported discrepancies between the effects of acetylcholine on various cardiac parameters. This information will be of importance in delinating more clearly the complex effects of parasympathetic activation on the heart. Further understanding of parasympathetic mechanisms involved in normal cardiac function (automacicity and conduction) is essential for understanding alterations in neural control evident in cardiac disease states.